Field of the Invention
The present invention relates to an image deformation technique.
Description of the Related Art
In many cases, an image processing apparatus is required to perform image transformation process. For example, a projector product performs image transformation process called keystone (trapezoidal) deformation processing. More specifically, when output light from the projector is projected on the screen, a valid area projected on the screen is deformed in a trapezoidal shape due to the installation tilt angle of the projector, an optical lens shift, or the like. It is hard for a user to view a valid area kept deformed in a trapezoidal shape. For this reason, image transformation process is performed to deform the valid area into an inverted trapezoidal shape, thereby deforming the valid area projected on the screen into a rectangular shape. This image transformation process is generally known as keystone (trapezoidal) deformation processing.
In order to perform image transformation process, it is necessary to perform the processing of referring to a plurality of pixels of an input image before deformation and generating each pixel of the image after the deformation by interpolating these pixels. As this technique, for example, a bicubic interpolating process is generally known. In a bicubic interpolating process, each pixel of an image after deformation is calculated by referring to 4×4 pixels of the image before the deformation and calculating the total sum of the products of the respective pixels and weighting coefficients defined in the bicubic interpolating process. A bicubic interpolating process has a sharpening effect and the effect of obtaining a clear image with enhanced edges after the process. On the other hand, however, this process has an undesired influence on an image originally having an edge before deformation. That is, the edge is excessively enhanced. This influence is generally known as ringing. Ringing appears on an image such that an excessively enhanced portion of the image looks different in color tint from the image before the deformation. As described above, ringing has a feature that it occurs more easily in a non-flat area (in other words, an area with an edge) of an image than in its flat area.
Japanese Patent No. 4762115 discloses a method of reducing ringing by changing the number of reference pixels in pixel interpolation processing in accordance with the flatness of each area of an image. More specifically, pixel interpolation processing with a large number of reference pixels, which has a high sharpening effect but easily causes ringing, is applied to a flat area in which ringing does not easily occur. On the other hand, pixel interpolation processing with a small number of reference pixels, which has a low sharpening effect but does not easily cause ringing, is applied to a non-flat area in which ringing easily occurs. Japanese Patent No. 4762115 suppresses the occurrence of ringing by switching the types of processing in this manner.
In addition, Japanese Patent No. 3706206 discloses a method of correcting the pixel values of reference pixels in pixel interpolation processing in accordance with the flatness of each area of an image. More specifically, pixel interpolation processing is performed for a flat area, in which ringing does not easily occur, without correcting the pixel values of reference pixels in pixel interpolation processing. On other hand, pixel interpolation processing is performed for a non-flat area, in which ringing easily occurs, upon correcting the pixel values of reference pixel in pixel interpolation processing so as to flatten the pixel values of the reference pixels. Japanese Patent No. 3706206 suppresses the occurrence of ringing by switching the type of processing in this manner.
Although Japanese Patent No. 4762115 discloses the method of changing the number of reference pixels in pixel interpolation processing in accordance with the flatness of each area of an image, pixel interpolation processing with a large number of reference pixels has a problem that the amount of processing increases in proportion to the square of one side of an area corresponding to the number of reference pixels. If, for example, the number of reference pixels is changed from 4×4 pixels to 8×8 pixels exemplified in Japanese Patent No. 4762115, the amount of processing increases at least four times in proportion to the square of one side of an area corresponding to the number of reference pixels, resulting in an increase in implementation cost.
Japanese Patent No. 3706206 discloses the method of correcting the pixel values of reference pixels in pixel interpolation processing in accordance with the flatness of each region of an image. This method, however, has a problem that in a non-flat area, in which ringing easily occurs, color tint changes before and after pixel interpolation processing regardless of the influence of ringing.